The message: "y_min should not equal y_max!" appears when plotting binary data included in the gnuplot file (plot '-').
To reproduce the bug, use the attached plot file, zoom on any portion of the graph and then zoom out (using 'p').
One can then zoom in but zooming out is not possible anymore.
I tried with gnuplot 4.6 (gp460win32.zip) and a binary version of gnuplot 4.7 (september 16) on windows 7 64bits
Hope it helps !